1. Field of the Invention
The present invention relates to a video signal format converting circuit for generating a video signal of a video signal format other than a composite video signal from a digital video signal that is reproduced from a digital VCR, received by a digital satellite broadcasting television receiver or a digital cable television receiver, or the like and for modifying the video signal so as to protect it from being copied.
2. Description of the Related Art
In recent years, as digital signal processing technologies have advanced, digitization of video signals has been actively employed for digital VCRs and digital satellite broadcasting receivers. In addition to digital video apparatuses that handle composite video signals, other digital video apparatuses that handle so-called S terminal output signals, or a luminance signal and color difference signals (YUV signals), or RGB signals have been placed in the market so as to improve image quality of programs.
On the other hand, since apparatuses as represented by VCRs that record and reproduce video signals have been widely used, copy protecting methods such as video signal modifying method by Macro Vision Company (U.S. Pat. Nos. 4,631,603, 4,577,216, 4,819,098, and 5,315,448) have been employed from a point of view of copyright protection for visual software packages. In a conventional analog VCR, a particular signal that causes the operation of the AGC circuit of a recording VCR that dubs video signals of a movie video tape to malfunction is placed in vertical blanking intervals of the video signals. The resultant video signals are output from the VCR. However, in the digital VCR and digital broadcasting television receiver, information in the vertical blanking intervals is not recorded and broadcast so as to reduce the data amount. In these apparatuses, when reproduced or received digital video signals are converted into standard television signals such as NTSC television signals, a synchronous signal or the like is added thereto. Thus, the signal that causes the operation of the AGC circuit of the abovedescried recording VCR to malfunction is added along with the synchronous signal or the like.
Thus, it is necessary to convert a received digital video signal into a signal format corresponding to other than the composite signal format so as to handle digital video apparatuses such as high image quality digital VCRs and digital broadcasting television receivers. In addition, when output signals other than the composite signals are not protected by a copyright law, it is improper from a view point of the protection of intellectual property rights. Consequently, it is necessary to modify video signals corresponding to other than the composite signals so as to protect them from being copied.